Synthesis and photoisomerization of enediyne-type dendrimers with benzyl ether dendrons

Article abstract of DOI:10.1246/bcsj.80.1995

Dendrimers with enediynes at the core and a benzyl ether-type dendron at the surrounding periphery were prepared as pure cis- and trans-isomers. Intramolecular energy-transfer efficiency and emission properties as well as photoiso-merization around the C=C double bond were studied.

Full text of DOI:10.1246/bcsj.80.1995

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 10, 1995–2000 (2007) 1995
Synthesis and Photoisomerization of Enediyne-Type
Dendrimers with Benzyl Ether Dendrons
Nao Yoshimura,1 Atsuya Momotake,1 Yoshihiro Shinohara,2
ꢀ1
Yoshinobu Nishimura,1 and Tatsuo Arai
1Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
2Research Facility Center for Science and Technology, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
Received April 6, 2007; E-mail: arai@chem.tsukuba.ac.jp
Dendrimers with enediynes at the core and a benzyl ether-type dendron at the surrounding periphery were prepared
as pure cis- and trans-isomers. Intramolecular energy-transfer efficiency and emission properties as well as photoiso-
merization around the C=C double bond were studied.
Lots of studies have been reported on the photoisomeriza-
tion and photophysical properties of simple olefins, such as
Result and Discussion
stilbenes and polyenes.1–3 Besides the extensive studies on
olefinic compounds, relatively few have involved the photo-
chemistry of enediyne-type compounds.4–6 Enediyne com-
pounds may undergo a thermal cyclization reaction to give
the di-radicals as reactive intermediates.7–11 Photochemical
cyclization reaction of some of the enediyne molecules have
also been reported.12–16 Because of the lack of basic research
on the photochemical properties of enediyne compounds, we
have studied and reported the photochemical behaviors of
enediynes having phenyl substituents with and without
donor–acceptor substituents on the both end of the triple
bonds.5,6
We have been interested in the effect of large substituent
on the photochemical conformational change of the nano-
second timescale by synthesizing dendrimers with the substitu-
ents having different generation dendrons of hydrophobic or
hydrophilic properties. Among them, stilbene-cored dendri-
mers with hydrophobic benzyl ether-type dendron undergo
trans–cis isomerization quite efficiently even for the higher
generation dendrimers.17 Thus, the dendron group as a large
substituent scarcely suppresses the photoisomerization effi-
ciency of stilbene. By introducing a hydrophilic substituent,
such as carboxylate group at the periphery, the surrounding
dendron caused a change in the photochemical behavior of
the core stilbene to induce almost one-way trans–cis isomeri-
zation.18,19
We have already found and reported that both cis and trans
isomers of enediyne compounds with phenyl group at the both
end of carbon–carbon triple bond exhibit fluorescence emis-
sion and undergo intersystem crossing to the triplet state to
give the triplet lifetime of ca. 300 ns. In addition, photochem-
ical cis–trans isomerization was observed. In this study, we
are focused on the effect of benzyl ether-type dendron on the
photochemical processes both in the excited singlet state and
triplet state. We report here the fluorescence, isomerization
and the triplet state behaviors of dendrimer structure of ene-
diyne compounds.
Synthesis. The dendrimers cis- and trans-G1–G4 were
prepared as illustrated in Scheme 1, and their structures were
identified by MALDI-TOF-Mass and NMR spectroscopes.
Optical Properties. Figure 1 shows the absorption, fluo-
rescence, and fluorescence excitation spectra of G1–G4 in
THF at room temperature. The absorption band around 300–
ꢀ
350 nm was mainly due to the ꢀ–ꢀ band of the enediyne unit,
and the band at shorter wavelength region around 280 nm was
ascribed to the benzyl ether-type dendrons. The values of the
extinction coefficient (") at 331 nm were 2:1 ꢁ 104 and 3:9 ꢁ
104 cmꢂ1 Mꢂ1 for cis- and trans-G1, respectively. Although the
absorption spectra of G2–G4 were almost the same at longer
wavelength region, the absorbance at 280 nm increased with
increasing generation due to the absorption spectra of benzyl
ether-type dendron group. The fluorescence quantum yield of
trans-G1–G4 and cis-G1–G4 on excitation at 320 nm was
determined to be 0.59–0.66 and 0.34–0.38 (Table 1), respec-
tively, at room temperature. These results indicate that the ex-
cited singlet states of cis- and trans-G1–G4 exist as the energy
well to be able to emit fluorescence. The fluorescence excita-
tion spectra in Fig. 1 showed a peak at 280 nm, indicating
that the energy transfer from the surrounding dendron group
to the core enediyne took place efficiently. The efficiency of
this energy-transfer process was estimated from the compari-
son of the absorption spectra and fluorescence excitation spec-
tra to be 86 and 73% for cis- and trans-G3, 58 and 78% for cis-
and trans-G4, respectively. Interestingly, even the higher
generation dendrimer G4 showed energy transfer efficiency
of more than 50%. Fluorescence lifetime was measured to be
1 ns for the trans isomers. The fluorescence lifetime of cis
isomer was shorter than the detection limit of our instrument
to be <0:5 ns. Thus, the generation scarcely affected the fluo-
rescence lifetime.
Figure 2 shows the change in the absorption spectra on
excitation at 332 nm in THF. The absorbance changed to
give the photostationary state of trans-to-cis isomer ratios of
52/48, 51/49, 53/47, and 45/55 for G1, G2, G3, and G4,
Published on the web October 12, 2007; doi:10.1246/bcsj.80.1995

1996 Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007)
Properties of Enediyne-Type Dendrimers
Scheme 1. a) cis-1,2-Dichloroethene, butylamine, Pd(PPh)2Cl2, CuI, benzene. b) trans-1,2-Dichloroethene, butylamine, Pd(PPh)2-
Cl2, CuI, benzene. c) Benzyl bromide for cis- and trans-G1, 3,5-bis(benzyloxy)benzyl bromide for cis- and trans-G2, 3,5-bis[3,5-
bis(benzyloxy)benzyloxy]benzyl bromide for cis- and trans-G3, and 3,5-bis[3,5-bis[3,5-bis(benzyloxy)benzyloxy]benzyloxy]-
benzyl bromide for cis- and trans-G4, respectively, K2CO3, THF–DMF (3:1).
Fig. 1. Absorption (thin line), fluorescence (solid line), and fluorescence excitation spectra (dash line) of cis-G1–G4 (a) and
trans-G1–G4 (b) in THF.

N. Yoshimura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007) 1997
Fig. 2. Change in the absorption spectra of trans-G4 (a) and cis-G4 (b) upon irradiation at 332 nm in THF under Argon. Compari-
son of the absorbance change during the irradiation, monitored at 332 nm for trans-G1–G4 (c) and cis-G1–G4 (d).
tem crossing from the singlet excited state of cis-isomer to the
triplet excited state is promoted by oxygen, and in the excited
triplet state, the cis-isomer undergoes isomerization with high
efficiency, producing the trans isomer in the ground state.
Usually, the singlet excited state of aromatic compounds is
quenched by molecular oxygen with a rate constant of 2 ꢁ
1010 Mꢂ1 sꢂ1 (kq) in organic solvents,20 and the excited singlet
state of cis- and trans-G1 is expected to be quenched by oxy-
gen with kq½O2ꢃ ¼ 3 ꢁ 108 sꢂ1. The deactivation rate constant
of the excited singlet state of cis-G1 is the inverse of the sin-
glet lifetime and must be faster than 2 ꢁ 109 sꢂ1 (¼1=ꢁs) and
is one order of magnitude faster than that of kq½O2ꢃ ¼ 3 ꢁ
108 sꢂ1. Thus, about 10% of the excited singlet state of cis-
G1 may be quenched by oxygen under oxygen atmosphere.
Oxygen induced the intersystem crossing from the singlet ex-
cited state to the triplet excited state, in which the isomeriza-
tion took place to give the 50% trans isomer from the resulted
cis triplet excited state. The quantum yield of cis-to-trans iso-
merization of cis-G1 was 0.30 in the absence of oxygen under
argon atmosphere. If 10% of the excited singlet state of cis-G1
is quenched by oxygen to give the triplet excited state, in
which deactivation occurs to yield the trans isomer with a
quantum yield of 0:1 ꢁ 0:5 ¼ 0:05, this value is ca. 20%, and
therefore, the increase in the quantum yield of cis-to-trans
isomerization can be explained by the acceleration of the inter-
system crossing by oxygen.
Table 1. Quantum Yields of Fluorescence and Photoiso-
merization for cis-G1–G4 and trans-G1–G4
cis-Isomer
trans-Isomer
ꢀf
ꢀc!t
ꢀf
ꢀt!c
G1
G2
G3
G4
0.38
0.37
0.37
0.34
0.30
0.32
0.32
0.26
0.63
0.59
0.59
0.66
0.18
0.19
0.18
0.19
respectively. The values of the quantum yield of cis-to-trans
and trans-to-cis photoisomerization (ꢀc!t and ꢀt!c) were
not affected by the generation (Table 1) indicating that the
large dendron group scarcely affected the isomerization in
the excited state. The cis–trans isomerization needs the twist-
ing of the double bond to give the perpendicular conformation
in the excited state, followed by deactivation to the ground-
state perpendicular conformation and further twisting to give
the isomer or reacting back to the starting conformation. The
lack of the effect of dendrimers on the photoisomerization
indicates that the large dendron substituent does not affect
the twisting around the double bond to give the twisted excited
state.
The fluorescence intensities of G1 and G4 were reduced
by the presence of oxygen. In this quenching process, oxygen
accelerates intersystem crossing to the triplet state, in which
the isomerization takes place. For example, the efficiency of
cis-to-trans photoisomerization of cis-G1 under oxygen was
0.36, which was ca. 20% higher than that under argon atmo-
sphere (ꢀc!t ¼ 0:30, Table 1). Thus, oxygen accelerates the
cis-to-trans isomerization by the following processes. Intersys-
GPC Analysis. Although the cis and trans isomers of
azobenzene-type dendrimers have been reported to be separat-
able by GPC, ultrafast non-radiative deactivation as well as
the lack of fluorescence emission disturbs the study of funda-
mental photochemical properties of azobenzene dendrimers.

1998 Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007)
Properties of Enediyne-Type Dendrimers
Fig. 3. (a) GPC chromatograms of the cis and trans mixtures of G1–G4 dendrimers (solid line). Overlay of GPC chromatograms of
cis- and trans-isomer (dashed line). (b) GPC chromatograms of the cis-G1–G4 mixtures and (c) trans-G1–G4 mixtures.
The present endiyne-type compounds exhibited fluorescence
emission with considerably high quantum yield. If separation
can be achieved by GPC, the photochemical isomerization
processes can more easily be followed. In this respect, we have
investigated the GPC profile of the dendrimers.
change in the size and the molecular structure of the dendritic
compounds may be used to prepare and to construct molecules
with new emissive and/or reactive properties.
Experimental
Solvents and commercially available compounds were pur-
chased from standard suppliers and purified by standard methods.
1H NMR spectra were measured with a Bruker ARX-400 (400
Figure 3 shows the HPLC data with GPC column. In the
lower generation dendrimer, cis and trans isomer could be
separated to give two peaks with retention times of 9.61 and
9.45 min for cis- and trans-G1, respectively. With an increase
in the generation, the retention time became shorter, and the
separation became less clear. However, the retention time of
the cis and trans isomers was still different even in G4 (7.60
and 7.50 min for cis- and trans-G4, respectively), and the cis
isomer came later indicating that the cis isomers in G1–G4
dendrimers have more compact structure than trans isomers.
The surrounding dendron groups are flexible, but the confor-
mation of the core enediyne group seems to control the molec-
ular size, of which the cis isomer seems to be smaller than
trans isomer.
1
MHz for H NMR) spectrometer in solution of CDCl3 with tetra-
methylsilane as an internal standard. UV absorption and fluores-
cence spectra were recorded on a Shimadzu UV-1600 UV–visible
spectrophotometer and on a Hitachi F-4500 fluorescence spec-
trometer, respectively. MALDI-TOF-MS measurements were
taken on using 2-(4-hydroxyphenylazo)benzoic acid (HABA) as
a matrix.
cis-2.
A mixture of cis-1,2-dichloroethene (164 mg, 1.69
mmol), copper(I) iodide (28 mg, 0.15 mmol), bis(triphenylphos-
phine)palladium dichloride (56 mg, 0.08 mmol), and butylamine
(349 mg, 4.7 mmol) and compound 121 (832 mg, 3.59 mmol) in
benzene (20 mL) were stirred at room temperature for 11 h. The
reaction mixture was quenched with water, and the organic layer
was separated. The aqueous phase was extracted with CH2Cl2
(50 mL), and the combined organic layer was dried over MgSO4,
filtered, and concentrated to dryness. The residue was purified by
silica-gel column chromatography (hexane/CH2Cl2 1:1) to give
cis-2 (580 mg 1.19 mmol) in 70% yield. 1H NMR (CDCl3; 400
MHz; Me4Si) ꢂ 7.22–7.13 (4H, m, ArH), 6.97 (2H, dd, J ¼ 1:5
Hz, J ¼ 4:0 Hz, ArH), 6.80 (2H, ddd, J ¼ 1:5 Hz, J ¼ 4:0 Hz, J ¼
8:0 Hz, ArH), 6.08 (2H, s, CH=CH), 0.97 (18H, s, C–CH3), 0.18
(12H, s, Si–CH3).
Conclusion
In summary, the enediyne-type dendrimer compounds were
successfully prepared for the first time as pure cis and trans
isomers. They underwent efficient photochemical isomeriza-
tion even with large dendron substituents. Furthermore, fluo-
rescence lifetime and the triplet lifetime were scarcely affected
by dendron substitution. However, the molecular structure of
the cis and trans isomers of higher generations of G4 were
a little different as revealed by GPC experiments. Thus, the

N. Yoshimura et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007) 1999
5.63; N, 0.00%.
trans-2. A mixture of trans-1,2-dichloroethene (149 mg, 1.54
mmol), copper(I) iodide (27 mg, 0.15 mmol), bis(triphenylphos-
phine)palladium dichloride (52 mg, 0.08 mmol), and butylamine
(312 mg, 4.23 mmol) and compound 1 (792 mg, 3.41 mmol) in
benzene (20 mL) were stirred at room temperature for 15 h. The
reaction mixture was quenched with water, and the organic layer
was separated. The aqueous phase was further extracted with
CH2Cl2 (50 mL), and the combined organic layer was dried over
MgSO4, and filtered, and the solvent was evaporated. The residue
was purified by silica-gel column chromatography (hexane/
CH2Cl2 20:1) to give trans-2 (192 mg, 0.39 mmol) in 25% yield.
1H NMR (CDCl3; 400 MHz; Me4Si) ꢂ 7.16 (2H, t, J ¼ 8:0 Hz,
ArH), 7.05 (2H, ddd, J ¼ 1:5 Hz, J ¼ 4:0 Hz, J ¼ 8:0 Hz, ArH),
6.92 (2H, dd, J ¼ 1:5 Hz, J ¼ 4:0 Hz, ArH), 6.80 (2H, ddd, J ¼
1:5 Hz, J ¼ 4:0 Hz, J ¼ 8:0 Hz, ArH), 6.25 (2H, s, CH=CH), 0.97
(18H, s, C(CH3)3), 0.19 (12H, s, Si(CH3)2).
trans-G2. 1H NMR (CDCl3; 400 MHz; Me4Si) ꢂ 7.44–7.29
(20H, m, ArH), 7.23 (2H, t, J ¼ 8:2 Hz, ArH), 7.09–7.03 (4H, m,
ArH), 6.96–6.91 (2H, m, ArH), 6.67 (4H, d, J ¼ 2:0 Hz, ArH),
6.58 (2H, t, J ¼ 2:0 Hz, ArH), 6.28 (2H, s, CH=CH), 5.04 (8H,
s, CH2OAr), 5.00 (4H, s, CH2OAr); MALDI-TOF-MS Calcd for
C60H48O6 ½M þ Naꢃþ ¼ 887:4; Found 886.5.
trans-G3. 1H NMR (CDCl3; 400 MHz; Me4Si) ꢂ 7.43–7.28
(40H, m, ArH), 7.21 (2H, t, J ¼ 8:0 Hz, ArH), 7.07–7.04 (4H, m,
ArH), 6.95–6.91 (2H, m, ArH), 6.69–6.64 (12H, m, ArH), 6.58–
6.53 (6H, m, ArH), 6.25 (2H, s, CH=CH), 5.03 (16H, s, CH2OAr),
4.97 (8H, s, CH2OAr); MALDI-TOF-MS Calcd for C116H96O14
½M þ Naꢃþ ¼ 1735:7; Found 1736.2.
trans-G4. 1H NMR (CDCl3; 400 MHz; Me4Si) ꢂ 7.40–7.25
(80H, m, ArH), 7.23–7.16 (2H, m, ArH), 7.05–7.01 (4H, m, ArH),
6.92–6.87 (2H, m, ArH), 6.67–6.63 (28H, m, ArH), 6.56–6.52
(14H, m, ArH), 6.22 (2H, s, CH=CH), 4.99 (32H, s, CH2OAr),
4.94 (16H, s, CH2OAr). MALDI-TOF-MS Calcd for C228H192O30
½M þ Kꢃþ ¼; 3449.4; Found 3452.8.
cis-G1 (Typical Procedure). A mixture of cis-2 (204 mg,
0.42 mmol), K2CO3 (201 mg, 1.45 mmol), 18-crown-6 ether (38
mg, 0.15 mmol), and benzyl bromide (158 mg, 0.92 mmol) in
dry acetone (30 mL) were refluxed under nitrogen for 2 h. After
evaporation to dryness, the residue was dissolved in dichloro-
methane and washed with water. The organic layer was dried over
MgSO4, filtered, and concentrated. The mixture was purified by
silica-gel column chromatography (hexane/ethyl acetate 5:1) to
give cis-G1 as a white solid (112 mg, 0.25 mmol, 60% yield):
1H NMR (CDCl3; 400 MHz; Me4Si) ꢂ 7.41–7.30 (10H, m, ArH),
7.24 (2H, t, J ¼ 8:2 Hz, ArH), 7.16–7.13 (4H, m, ArH), 6.96 (2H,
ddd, J ¼ 1:2 Hz, J ¼ 2:4 Hz, J ¼ 8:2 Hz, ArH), 6.10 (2H, s, CH=
CH), 5.01 (4H, s, CH2OAr); Anal. Calcd for C32H24O2: C, 87.25;
H, 5.49; N, 0.00%. Found: C, 87.08; H, 5.67; N, 0.00%.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417), a Grant-in-Aid for Scientific
Research (No. 16350005) and the 21st Century COE Program
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of the Japanese Government, by Univer-
sity of Tsukuba Research Projects, Asahi Glass Foundation
and JSR Corporation.
References
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1
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0.18 mmol), K2CO3 (100 mg, 0.72 mmol), 18-crown-6 ether
(23 mg 0.09 mmol), and benzyl bromide (90 mg, 0.62 mmol) in
dry acetone (30 mL) were refluxed under nitrogen for 17 h. After
evaporation to dryness, the residue was dissolved in dichloro-
methane and washed with water. The organic layer was dried over
MgSO4, filtered, and concentrated. The mixture was purified by
silica-gel column chromatography (hexane/CH2Cl2 4:1) to give
trans-G1 as a white solid (62 mg, 0.14 mmol, 78%): 1H NMR
(CDCl3; 400 MHz; Me4Si) ꢂ 7.47–7.32 (10H, m, ArH), 7.25 (2H,
t, J ¼ 8:2 Hz, ArH), 7.11–7.06 (4H, m, ArH), 6.99–6.95 (2H, m,
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2000 Bull. Chem. Soc. Jpn. Vol. 80, No. 10 (2007)
Properties of Enediyne-Type Dendrimers
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